The term synthetic aperture radar (SAR) is often used to describe radar systems that use a moving antenna to simulate an extremely large antenna or aperture electronically. SAR systems are often mounted to airborne or space-based platforms and are mounted at an angle relative to the flight path of the platform to which the antenna is mounted. A monostatic SAR utilizes the same platform for the transmitter and receiver.
The flight path of the platform on which a SAR is mounted defines the azimuth direction with the antenna generally focused on a direction orthogonal to the azimuth (See FIG. 1). The direction in which the antenna is directed is often referred to as the range or slant-range. If the direction of observation is perpendicular to the direction of travel, the system is defined as a boresight system. Otherwise it is called a squinted system.
SARs can be configured in strip map mode or spotlight mode. In strip map mode, the SAR transmits coherent pulses in a fixed pointing direction of the radar antenna as the platform moves. In spotlight mode, the SAR steers the radar beam to keep a target within the beam for a longer time. As length of synthetic aperture increases, the azimuth resolution increases. However, the area sampling rate is reduced in spotlight mode relative to the area sampling rate of a strip map mode SAR system that transmits over the same time period.
In a SAR system, data is acquired by transmitting a radio pulse and receiving a signal backscattered by the imaged scene. In such systems, resolution in range increases with the bandwidth of the transmitted pulse. In many systems a frequency modulated pulse, referred to as a chirp, that is a linear frequency sweep is utilized to achieve high resolution without decreasing pulse duration. Chirps are interspersed with quiescent periods for reception. In polarimetric SAR systems, chirps are typically transmitted with alternating polarities.
In a strip map SAR system, azimuth resolution is typically dependent upon the length (or effective dimension) of the antenna. Reducing the length of the antenna increases azimuth resolution. Discrimination of targets based upon azimuth position is possible due to phase variation during the observation period. When the Doppler bandwidth is under-resolved (i.e. the pulse repetition rate is below the Nyquist rate of the anticipated Doppler bandwidth), aliasing can occur. Formation of a synthetic aperture without aliasing typically requires transmission of radar pulses at along track distances equal to or less than half the length of the antenna. Accordingly, the antenna length and the velocity of the platform to which the SAR is mounted determines a desired pulse repetition rate of the radar system to avoid aliasing.
There are two competing constraints on the pulse repetition rate of a SAR system. On one hand, if the pulses are transmitted too frequently, then reflections from more than one range will arrive at the receiver simultaneously, leading to range (cross-track) ambiguity. On the other, if pulses are not transmitted frequently enough, then the Doppler bandwidth will be under-resolved, leading to azimuth (along-track) aliasing and ambiguity.
For space-based SAR systems, orbital speed can be so high that a large antenna is required to enable a pulse repetition rate that is sufficiently low to avoid range ambiguity. Increasing the size of the antenna reduces azimuth resolution in strip map mode. The decrease in resolution can be offset by operating in spotlight mode with a consequent loss of collection area rate. Often, the resulting physical antenna is longer than signal-to-noise considerations alone would otherwise require. The requirement of a large antenna, coupled with typically tight antenna RF precision requirements, typically means that either an expensive rocket with a large payload fairing is required, or a large, high-precision structure must be unfurled in space, raising engineering and manufacturing costs and mission risk. And indeed, space-based SAR missions launched to date typically feature either a long, often segmented antenna, or a large deployed parabolic dish.